A metal oxide semiconductor field effect transistor (MOSFET) has high input impedance and thus has a very simple gate driving circuit compared to a bipolar transistor. Thus, it may have an advantage in that there is no time delay caused by accumulation or recombination of minority carriers while the device is turned-off because the MOSFET is a unipolar device.
The MOSFET is used for a switching mode power supply, a lamp ballast, and a motor driving circuit. A drain extended MOSFET structure using planar diffusion technology has been used for a MOSFET for power.
A trench gate-type MOSFET structure formed by etching a semiconductor substrate to a predetermined depth to form a trench, and filling the trench with a gate conductive layer is under study. The trench gate-type MOSFET increases cell density per unit cell, and decreases the resistance of a junction FET (JFET) between devices, thereby realizing high integration and small source-drain-on resistance Rds (on).
A trench forming a cell of the trench gate type MOSFET has a stripe shape or a cross shape. In a stripe-shaped trench gate, an N+ source region (or P+ source region) and a P type body region (or an N type body region) are connected to each other along the sidewall of the trench.
In a stripe-shaped trench structure, a voltage applied to a source region and a body region is uniformly distributed along the trench. However, since density of cell is low compared to a cross type trench structure, on-resistance may be large.
In a cross-shaped trench structure, an N+ source region (or P+ source region) and a P-type body region (or N type body region) are isolated in the trench, so that they are electrically connected to each other by only a contact material between the source and the body. In the trench gate-type MOSFET, density of a cell is important to secure a large region where a drain current flows per unit area.